In a high-speed digital bus, the Input/Output (“I/O”) buffer edge-rate is an essential design parameter. If the buffer edge rate is not controlled within design specifications, then signal integrity issues such as inter-symbol interference (“ISI”) may occur. ISI, like other signal interferences, causes degradation in system performance at higher data rates. ISI is particularly menacing because raising the signal power does not decrease the probability of making a bit error. Thus, in high speed digital transmitters, the buffer edge rate must be controlled to maintain data transfer bandwidth.
The characteristics of a pre-driver circuit used in concert with a driver circuit often determine the I/O buffer edge-rate. Usually, the I/O buffer edge-rate is a function of the pre-driver slew-rate. The slower the pre-driver slew-rate, as compared to the driver slew-rate, the more likely that ISI issues will be created when the driver switching frequency increases and the buffer edge rate remains unchanged. This often occurs when the driver bandwidth is greater than the pre-driver bandwidth. When the switching frequency is higher than the pre-driver bandwidth, the pre-driver signal swing will not be rail-to-rail. On the other hand, when the switching frequency is less than the pre-driver bandwidth, the pre-driver signal swing will be rail-to-rail. Thus, the signal swing and therefore the switching time, for the low frequency signals is greater than signal swing, and therefore the switching time, for the high frequency signals. These two different pre-driver signal swings, and their resulting switching times may result in the driver skew for an HL (High-to-Low) transition being different than the driver skew for a LH (Low-to-High) transition, which causes signal distortion and potentially frequency dependent ISI signal integrity issues. Therefore, ideally, these two skews would be approximately the same.
An ideal pre-driver circuit would operate for both low frequency and high frequency data signals in a high speed digital transmission environment without generating excessive ISI problems as well as other signal integrity issues.